1. Field of the Invention
The present invention relates to a brake-fluid reflux type anti-skid brake control system employing a brake-fluid flow control valve controls a flow rate of brake fluid introduced therethrough into the wheel cylinder in response to a pressure difference between a fluid pressure of a master-cylinder side and a fluid pressure of a wheel-cylinder side, and a fluid-pressure pump refluxing the brake fluid in the wheel cylinder just downstream of the master cylinder during an anti-skid brake control at a wheel-cylinder pressure reducing mode at which the wheel-cylinder pressure is reduced, and specifically to a system employing a brake-fluid flow control valve through which in a wheel-cylinder pressure is moderately intensified during operation of the anti-skid brake control system in a wheel-cylinder pressure intensifying mode at which the wheel-cylinder pressure is increased.
2. Background of the Prior Art
Recently, there have been proposed and developed various automotive brake control systems, such as an anti-skid brake control system (generally abbreviated as an "ABS") which is provided for preventing brakes from locking road wheels during quick braking or during braking on a low frictional road surface so as to prevent skidding. In large luxury cars and high-grade vehicles, the ABS is often assembled to provide maximum effective braking, irrespective of the road condition. The ABS permits normal application of the brakes by alternately reducing and intensifying the wheel-cylinder pressure so that braking can be held to just below the point at which a skid would start to develop.
As is generally known, the ABS comes into operation if an electronic control unit detects whether a vehicle wheel is locked during quick braking or braking on a low frictional road surface. At the beginning of the anti-skid brake control, the ABS operates at a wheel-cylinder pressure reducing mode which will be referred to as an "ABS pressure reducing mode". In the ABS pressure reducing mode, the brake fluid in the wheel cylinder is temporarily stored in a brake fluid reservoir through a directional control valve serving as a pressure reducing valve. Thereafter, a fluid-pressure pump is driven with a slight time lag in order to reflux the brake fluid stored in the reservoir to a hydraulic pressure supply line connected to the outlet port of the master cylinder, with the result that the wheel-cylinder pressure is gradually reduced and thus the pressure difference between the fluid pressure of the master-cylinder side and the fluid pressure of the wheel-cylinder side is greater than or equal to a predetermined threshold. Since the fluid-pressure pump is driven only during operation of the ABS, the fluid pump is generally referred to as an "ABS pump". In this manner, the wheel-cylinder pressure is reduced. Consequently, as soon as the control unit detects that the vehicular wheel has been unlocked during operation of the ABS at the ABS pressure reducing mode, a brake fluid introduced from the outlet port of the master cylinder and/or a brake fluid discharged from the ABS pump and refluxed just downstream of the outlet port of the master cylinder are both fed through the directional control valve serving as a pressure intensifying valve to the wheel cylinder. Thus, the wheel-cylinder pressure is increased again.
As set forth above, the wheel-cylinder pressure is suitably adjusted by the ABS so as to effectively prevent an undesirable wheel-lock during quick braking or braking on a low friction road, while providing reasonably high braking force. After the wheel-cylinder pressure control mode is changed from the ABS pressure reducing mode to the ABS pressure intensifying mode, in the above-noted older ABS, there is a tendency for the brake fluids output from the master cylinder and the ABS pump to be rapidly fed into the wheel cylinder. There is a possibility that the wheel is locked again, because of an excessive pressure-rise. In order to solve such a problem, a more recent anti-skid brake control system has a brake-fluid flow control valve arranged in the hydraulic pressure supply line connected to the master cylinder and supplying the fluid pressure to the wheel cylinder. Such a brake-fluid flow control valve operates, so that during operation of the ABS, the flow control valve fully opens when the pressure difference between the fluid pressure of the master-cylinder side and the fluid pressure of the wheel-cylinder side is less than a predetermined threshold, and the flow control valve fully throttles the fluid flow therethrough when the pressure difference is greater than or equal to the threshold. Thus, a gradient of the pressure-rise in the wheel cylinder can be maintained at a relatively low level in the ABS pressure intensifying mode so as to suppress an excessive pressure-rise and to assure a moderate pressure-rise in the wheel cylinder. In other words, such a more recent ABS exhibits superior moderate wheel-cylinder pressure intensifying characteristics during operation. One such ABS has been disclosed in Japanese Utility Model First Publication (Jikkai) Heisei No. 3-2870.
The ABS employing a brake-fluid flow control valve is advantageous to reliably avoid wheel-lock. However, since the brake-fluid flow control valve is designed to fully throttle a brake-fluid passage area when the pressure difference between the fluid pressure of the master-cylinder side and the fluid pressure of the wheel-cylinder side is greater than or equal to the predetermined threshold value, the flow control valve may operate to provide a restricted fluid flow through an orifice constriction, even during quick braking where the vehicular wheel is not yet locked and the ABS is deactivated. In this case, an effective quick braking effect cannot be obtained, i.e., a quick braking response is deteriorated, because of flow restriction of the flow control valve.
In addition to the above, the conventional ABS is usually constructed, such that in four-wheeled vehicles, one of four brake-fluid flow control valves employed in the ABS is associated with each one of four wheels, namely a front-left wheel, a front-right wheel, a rear-left wheel and a rear-right wheel, in order to obtain an optimal moderate wheel-cylinder pressure intensifying characteristics with regard to each wheel cylinder, during an anti-skid brake control. This results in high manufacturing costs in four-wheeled vehicles having an ABS with four brake-fluid flow control valves. As will be appreciated from the above, it is more desirable that only one brake-fluid pressure control valve is applied commonly to a plurality of wheel cylinders, such as right and left wheel-cylinders, in such a manner as to obtain identical moderate wheel-cylinder pressure intensifying characteristics in respective wheel cylinders during operation of the ABS and to insure a high braking response during quick braking where the vehicular wheel is not yet locked under a deactivated condition of the ABS.